Summer and Winter Differences in Zooplankton Biomass
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Pretorius, M. et al. (2016). Summer and winter differences in zooplankton biomass, distribution and size composition in the KwaZulu-Natal Bight, South Africa African Journal of Marine Science, 38: S155-S168 https://doi.org/10.2989/1814232X.2016.1144650 Summer and winter differences in zooplankton biomass, distribution and size composition in the KwaZulu-Natal Bight, South Africa M. Pretorius, J.A. Huggett and M.J. Gibbons Abstract Zooplankton biomass and distribution in the KwaZulu-Natal Bight were investigated in relation to environmental parameters during summer (January–February 2010) and winter (July–August 2010). Mean zooplankton biomass was significantly higher in winter (17.1 mg dry weight [DW] m–3) than in summer (9.5 mg DW m−3). In summer, total biomass was evenly distributed within the central bight, low off the Thukela River mouth and peaked near Durban. In winter, highest biomass was found offshore between Richards Bay and Cape St Lucia. Zooplankton biomass in each size class was significantly, negatively related to sea surface temperature and integrated nitrate, but positively related to surface chlorophyll a and dissolved oxygen. Zooplankton biomass was significantly related to bottom depth, with greatest total biomass located inshore (<50 m). Distribution across the shelf varied with zooplankton size. Seasonal differences in copepod size composition suggest that a smaller, younger community occupied the cool, chlorophyll-rich waters offshore from the St Lucia upwelling cell in winter, and a larger, older community occurred within the relatively warm and chlorophyll-poor central bight in summer. Nutrient enrichment from quasi-permanent upwelling off Durban and Richards Bay appears to have a greater influence on zooplankton biomass and distribution in the bight than the strongly seasonal nutrient input from the Thukela River. Introduction The east coast of South Africa is characterised by a narrow (c. 11 km) continental shelf with a steep slope that provides stability to the northern Agulhas Current (Schumann 1988; de Ruijter et al. 1999). The KwaZulu-Natal (KZN) Bight is a 160 km-long widening of the shelf between Cape St Lucia to the north and Durban to the south (Lutjeharms et al. 2000a; Figure 1). The shelf is approximately 50 km wide at its broadest part off the mouth of the Thukela (formerly Tugela) River, the largest river in the KZN province. Early oceanographic studies in this region focused mainly around Richards Bay (Gründlingh 1974; Pearce 1978; Pearce et al. 1978) or Durban (Pearce 1977; Schumann 1981, 1982; Anderson et al. 1988), with little research conducted over the bight itself. Nutrient concentrations off the KZN coast were first investigated by Oliff (1973) but the study was limited to the Richards Bay area. The first extensive hydrographic survey off the East Coast to include the KZN Bight was conducted in July 1989 (Lutjeharms et al. 2000a; Meyer et al. 2002). The St Lucia upwelling University of the Western Cape Research Repository [email protected] cell was identified as the main source of nutrients for the bight, with additional upwelling of nutrients in the core of a recurrent lee eddy, known as the Durban Eddy, at the southern end of the bight, off Durban (Meyer et al. 2002). Lutjeharms et al. (1989) showed that the St Lucia upwelling cell, located where the shelf widens along the path of the current at the northern end of the bight, occurs year round and brings cold nutrient-rich waters from the central water depths onto the shelf, influencing the physical water characteristics of the whole bight (Lutjeharms et al. 2000b). Nutrient-rich upwelled water between Cape St Lucia and Richards Bay has been shown to have a substantial influence on phytoplankton productivity over the whole bight, with chlorophyll a concentration in the bight ranging from 0.03 to 3.88 mg m–3 (Carter and Schleyer 1988). Chlorophyll a concentrations closest to the Cape St Lucia upwelling cell were recorded at 1.2 mg m–3 by Lutjeharms et al. (2000a), increasing to 1.5 mg m–3 to the south, then decreasing further south (<0.5 mg m–3). Barlow et al. (2008) found that, in the bight, chlorophyll a concentrations peaked offshore to the north of Durban (2.8 mg m–3) and just south of Richards Bay (3.2 mg m–3), with lower concentrations near the coast ranging from c. 0.9 to 1.3 mg m–3. Most knowledge of East Coast zooplankton communities, in particular copepods, stems from a small number of once- off or widely spaced surveys conducted between the 1960s and 1980s, using a wide variety of sampling gear (De Decker 1964, 1984; De Decker and Mombeck 1964; Carter 1977; Schleyer 1985; Carter and Schleyer 1988). Zooplankton biomass on the continental shelf was up to an order of magnitude higher than in the Agulhas Current but was highly variable (mean 0.29 ml m–3; range 0.02–1.68 ml m–3), and with no evident seasonality (Carter and Schleyer 1988). 2 http://repository.uwc.ac.za The bulk (c. 70%) of the biomass was concentrated in the upper 100 m. Carter (1977) identified distinct neritic and oceanic copepod communities between Port Edward (160 km south of Durban) and St Lucia, and also noted the development of large populations of Calanoides carinatus, an upwelling species, in association with eddy centre upwelling off Durban. Carter and Schleyer (1988) found that copepod species assemblages varied seasonally, with communities dominated by Calanoides carinatus and Centropages chierchiae during winter/spring, when primary production was elevated, and by the smaller Paracalanus parvus during the other seasons, with a general increase in both copepod and chaetognath abundance during summer. Copepod diversity was highest within the core of the Agulhas Current and decreased to either side of it, with lowest diversity within the bight (De Decker 1984). Although Schleyer (1985) failed to observe any distinct communities of chaetognaths off Durban, Thibault-Botha et al. (2004) observed two aseasonal assemblages of siphonophores along the east coast of South Africa: one associated with the nearshore waters of the Port Alfred upwelling cell (characterised by low overall diversity) and the second with the inshore waters north of East London, which showed some evidence of alongshore and crossshelf structure. During the KZN sardine run each winter, large schools 3 http://repository.uwc.ac.za of sardine Sardinops sagax move east and northwards along the Transkei and KZN continental shelf, using cool-water conditions adjacent to the East Coast (O’Donoghue et al. 2010; van der Lingen et al. 2010). Beckley and Hewitson (1994) also found that larvae of sardine, round herring Etrumeus whiteheadi (probably E. teres [Connell 2001], now E. wongratanai) and anchovy Engraulis encrasicolus extended as far north as the mouth of the Thukela River. These authors observed that high larval abundance coincided with an upwelling node off Algoa Bay (near Port Elizabeth), and speculated that the same may be the case for Cape St Lucia. Understanding variability in zooplankton abundance is of general ecological interest as there has been very little zooplankton research in the KZN Bight, but also given the relevance of zooplankton in the diets of juvenile pelagic fish species such as sardine, which contribute to a commercially important fishery in South Africa. This study tests the hypotheses that (a) zooplankton biomass is elevated in (or downstream from) the three areas of local nutrient enrichment: (i) topographically-driven upwelling at Cape St Lucia; (ii) upwelling associated with the persistent cyclonic gyre off Durban (Schumann 1988; Lutjeharms et al. 2000a); and (iii) riverine inputs from the Thukela River (Schumann 1988), (b) zooplankton biomass varies seasonally, and is higher during summer (the wet season) due to higher chlorophyll a concentrations anticipated during this season as a result of nutrient run-off, and (c) zooplankton biomass varies with depth, being greatest inshore due to greater nutrient and chlorophyll a concentrations anticipated in that region, and declines offshore. Material and methods This study forms one component of an African Coelacanth Ecosystem Programme (ACEP) project, entitled Ecosystem Processes in the KwaZulu-Natal Bight, which was developed to investigate physical, geological and biological processes that drive the marine ecosystem in the bight, with a particular focus on the nutrient sources. Zooplankton samples were collected during two research surveys in the bight, in January/February 2010 (during the summer wet season) and July/August 2010 (during the winter dry season). Surveys of zooplankton biomass in the bight were conducted during a synoptic leg at the beginning of each research survey, and consisted of 16 transects, 15 of which extended perpendicular to the coast, from half a degree south of Durban to Cape St Lucia in the north (Figure 1). Hydrographic parameters were sampled at all 119 stations, whereas zooplankton samples were collected at alternate stations (c. 60 stations). At hydrographic stations, temperature, salinity and oxygen were profiled during CTD deployments, and seawater samples were collected at selected depths, according to the fluorescence profile, for analysis of chlorophyll a and nutrients (nitrite, nitrate, silicate and phosphate). Water samples for chlorophyll a analysis (500 ml) were filtered on board onto Whatman GF/F filters, which were then frozen and analysed later according to the fluorometric technique of Welschmeyer (1994). Nutrient samples were frozen on board and analysed ashore by standard auto analyser techniques (Mostert 1983). Integrated values of chlorophyll a and nutrients were calculated for the zooplankton sampling depth (upper 200 m) as well as for the whole water column beyond the continental shelf. During both research surveys, zooplankton samples were collected using bongo nets (200 μm mesh), which were hauled vertically from a maximum depth of 200 m, or several metres above the bottom.